Napping of a textile fabric is a well known technique for enhancing the aesthetic and performance characteristics of the fabric by raising the fabric surface to provide, among other things, a softer hand, improved drapeability, greater fabric thickness, and better durability. Various types of fabric are suitable for napping, in particular warp knitted fabrics wherein the underlaps formed at the technical back of the fabric are especially susceptible to being raised from the fabric surface.
Machinery for performing napping operations basically utilizes a rotatably driven cylinder having peripheral wire teeth, normally in the nature of card clothing, over which fabric to be napped travels in a tensioned condition. In the majority of conventional napping machines, the napping cylinder is of a so-called planetary type having a plurality of napping rollers covered with card clothing supported in spaced axially parallel relationship by planetary gear sets at opposite ends of the napping rollers, whereby the rollers form the outer circumferential periphery of the napping cylinder. The cylinder is driven to rotate in the same direction as the path of travel of the fabric being napped, while the planetary gear sets are operated to drive the napping rollers to rotate in opposition to the direction of cylinder rotation and fabric travel.
The napping rollers utilized in planetary-type napping cylinders may be of several differing constructions. Napping cylinders of the so-called double acting type have alternating napping rollers equipped with card teeth extending in the same direction as cylinder rotation, commonly referred to as pile rollers, and intermediate napping rollers equipped with card teeth extending in the opposite direction, commonly referred to as counter-pile rollers. In napping cylinders of the so-called single acting type, all of the napping rollers have card wire teeth which extend in the direction of cylinder rotation and fabric travel, i.e., only pile rollers. Knit goods action napping cylinders are equipped with alternating pile rollers and intervening rollers having straight card wire teeth which are neither inclined in the direction of fabric travel or counter thereto.
The napping effect achieved in any given napping machine is dependent on the type of napping cylinder employed, the speed at which the napping cylinder is operated with respect to the traveling speed of the fabric, the speed at which the napping rollers are driven in relation to the rotational speed of the napping cylinder, and a variety of other factors. With any type of napping cylinder, positive napping energy is achieved, i.e., the wire teeth of the napping rollers penetrate the fabric surface to achieve a raising effect, only when the napping rollers are driven at a greater peripheral speed than the peripheral speed of the overall napping cylinder, the amount of napping energy being related to the difference in their surface speeds. A double acting napping cylinder operated with positive napping energy produces a uniform non-directional pile surface on fabric being napped, while single acting napping cylinders operated with positive napping energy produce a pile surface having a characteristic unidirectional lay of the raised pile. Knit goods action napping cylinders achieve a napping effect similar to that of double acting napping cylinders and are primarily utilized for processing fleecy fabrics as a preliminary step to subsequent processing on other napping machines. In contrast, by driving the napping rollers of a napping cylinder at a lesser peripheral speed than that of the overall napping cylinder, i.e., with a negative napping energy, the wire teeth of the napping rollers act in a pushing rather than raising manner on the fabric surface to produce a so-called felting action.
In addition to the foregoing factors, the napping effect achieved by any given napping machine will vary from one fabric to another based on the type of fabric construction, the type of yarn utilized, the yarn count, yarn twist and like factors. The processes to which a fabric is subjected in advance of napping, particularly wet processing of the fabric, may also affect the napping operation. Furthermore, the napping effect will additionally vary without any change in the napping machine or the fabric being napped as a result of progressive wearing of the napping teeth over the course of machine operation.
Because of the interplay of all such variables, it is difficult if not impossible to predict the napping effect of any given napping machine on a given fabric. Accordingly, the operation of conventional napping machinery is widely viewed to be as much if not more of an art than a science, resting largely in the skill and experience of the machine operator. In conventional napping machinery, considerable time is required to change napping cylinders and further time is required of the machine operator to adjust the machine settings to obtain a desired napping effect. As a result, it is not economical to change the set-up of a napping machine very often and it is even more impractical to experiment to any significant degree with differing napping effects produced by differing napping cylinders operated at differing machine settings on differing fabrics.